My M9998 that's been on the repair bench for longer than I care to remember also had the FM stereo issue. There's several variable resistors on the FM board. I recorded their positions and deoxited them all. With a strong station tuned in, I then adjusted the one that Norm mentioned until all the strong stations worked properly. Luckily this resistor is very easy to reach just by taking off the back. I can take a picture if you want.
Secondly, the rebelting. Yeah, this is a disaster on this boombox. And it's not just the rebelting that may need to be done either. There's also an idler wheel with a rubber ring that may have dried up, it did in mine (broke and fell off). To get to that thing, you have to take the deck out of the chassis, requires desoldering of a switch, and that is quite the endeavor!
And then there's the cogs on the 2 spools of the tape deck. They are made of plastic that turns brittle over age, and are likely to disintegrate when you actually start using them. You can replace those with the cogs from other decks, e.g. Sharp GF-8989, pannies with soft touch control decks.
It has to be said though, *if* you have one that is in good condition, it does indeed sound amazing. But it's a total nightmare to service these.